The regulation of small nuclear RNA synthesis is being studied during early sea urchin development. The gene organization of the two major small nuclear RNAs in the sea urchin has been determined by Southern mapping. Each gene family is present in a tandem repeating unit of about 1400 bases. The repeating units are being isolated and cloned in the plasmid PBR 322. The synthesis of these RNAs is being studied in isolated sea urchin embryo nuclei. A nuclear system active in RNA synthesis has been prepared from embryos of different stages. Supplementing these nuclei with extracts from embryos at different stages alters the activity of the nuclei. A factor has been partially purified from sea urchin eggs which specifically inhibits RNA polymerase III in sea urchin embryo nuclei.